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Journal of Environmental Quality 30:1464-1473 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Wetlands and Aquatic Processes

Removal and Distribution of Iron, Manganese, Cobalt, and Nickel within a Pennsylvania Constructed Wetland Treating Coal Combustion By-Product Leachate

Z.H. Ye, S.N. Whiting, Z.-Q. Lin, C.M. Lytle, J.H. Qian and N. Terry*

Department of Plant and Microbial Biology, Univ. of California at Berkeley, 111 Koshland Hall, Berkeley, CA 94720

* Corresponding author (nterry{at}nature.berkeley.edu)

Received for publication May 31, 2000.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
A flow-through wetland treatment system was constructed to treat coal combustion by-product leachate from an electrical power station at Springdale, Pennsylvania. In a nine-compartment treatment system, four cattail (Typha latifolia L.) wetland cells (designated Cells 1 through 4) successfully removed iron (Fe) and manganese (Mn) from the inlet water; Fe and Mn concentrations were decreased by an average of 91% in the first year (May 1996–May 1997), and by 94 and 98% in the second year (July 1997–June 1998), respectively. Cobalt (Co) and nickel (Ni) were decreased by an average of 39 and 47% in the first year, and 98 and 63% in the second year, respectively. Most of the metal removed by the wetland cells was accumulated in sediments, which constituted the largest sink. Except for Fe, metal concentrations in the sediments tended to be greater in the top 5 cm of sediment than in the 5- to 10- or 10- to 15-cm layers, and in Cell 1 than in Cells 2, 3, and 4. Plants constituted a much smaller sink for metals; only 0.91, 4.18, 0.19, and 0.38% of the Fe, Mn, Co, and Ni were accumulated annually in the aboveground tissues of cattail, respectively. A greater proportion of each metal (except Mn) was accumulated in cattail fallen litter and submerged Chara (a macroalga) tissues, that is, 2.81, 2.75, and 1.05% for Fe, Co, and Ni, respectively. Considerably higher concentrations of metals were associated with cattail roots than shoots, although Mn was a notable exception.

Abbreviations: APS, Allegheny Power Services • EC, electrical conductivity • DO, dissolved oxygen • NPDES, National Pollution Discharge Elimination System


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
COAL-BURNING electric power generation facilities can encounter compliance difficulties when disposing of metal-contaminated wastewater discharges (Electric Power Research Institute, 1998). Such discharges are produced during the storage and processing of coal, and the storage and land disposal of coal ash residues produced during the combustion process. These discharges contain a variety of contaminants including metals, sulfur, and acid (low pH) (Kadlec and Knight, 1996), which seriously degrade water quality and biodiversity when released into streams and rivers. To comply with the National Pollution Discharge Elimination System (NPDES) effluent limitations, the contaminant load of the wastewater must be reduced. Conventional techniques of controlling metal-contaminated water (for example, chemical treatment) are costly in terms of equipment and chemicals. Moreover, they require intensive management and long-term maintenance (Brodie et al., 1993; Skousen et al., 1994). Estimated costs for the chemical treatment of waste solids and water produced by the U.S. coal industry exceed $1 million per day (Perry and Kleinmann, 1991).

Constructed wetland treatment systems offer a practical alternative to the chemical treatment of drainage from certain coal-related sources (Brodie et al., 1989; Nu-Hoai et al., 1998). The advantages include low costs of construction, operation, and maintenance, with effective removal of many contaminants such as heavy metals and metalloids (National Rivers Authority, 1992; Nu-Hoai et al., 1998). In addition, the systems are usually better able to cope with fluctuating water flow rate and variable contaminant concentrations than conventional treatment systems (Bastian and Hammer, 1993). The success of wetlands has resulted in the construction of more than 400 wetlands in the USA for the purpose of treating drainage water from coal mines alone (Perry and Kleinmann, 1991).

Wetlands are capable of removing large quantities of trace elements from wastewater passing through them. However, there is considerable variation among metals, and also between wetlands, in the degree to which each metal is removed by the wetland vegetation. For example, in a review of constructed wetlands in the USA, Skousen et al. (1994) reported that the removal of iron (Fe) from acid mine drainage varied widely between 28 and 99%. In the case of manganese (Mn), where the range of removal rates was between 8 and 98%, the mean removal rate was actually less than 30%. The mechanisms of removal for each contaminant from wastewater, and the interrelatedness of these different mechanisms, are not well understood (Faulkner and Richardson, 1989; National Rivers Authority, 1992). It is thought that the cleanup of water is accomplished through a variety of physical, chemical, and biological processes operating independently in some situations, and interactively in others (Dunbabin and Bowmer, 1992; Hammer, 1993).

Physicochemical mechanisms of metal retention in the sediments include immobilization via oxidative precipitation, sedimentation of metal-laden particulates, adsorption on cation exchange sites, complexation with organic matter, and sulfate reduction (Wieder, 1988; Perry and Kleinmann, 1991; Brodie et al., 1993). Precipitation of metal oxides, following microbially mediated oxidation, is thought to be one of the most important removal mechanisms in wetlands (National Rivers Authority, 1992; Skousen et al., 1994). Indeed, 40 to 70% of the total iron removed from acid mine drainage by some wetlands was found as ferric hydroxides from the hydrolysis of ferric iron or from microbial oxidation of ferrous iron (Henrot and Wieder, 1990).

Plants and microbes are important in many different ways for contaminant retention by wetlands. However, the direct uptake of trace elements into the shoots of plants represents only a minor proportion of the total removed by the wetland; for example, the bioaccumulation of Fe by plants, even those with a large biomass such as cattail, removed less than 1% of the Fe input to some constructed wetlands (Sencindiver and Bhumbla, 1988; Faulkner and Richardson, 1989; Mitsch and Wise, 1998; Whiting and Terry, 1999). Nevertheless, plants play a critical role in metal removal via filtration, adsorption, and cation exchange, and through plant-induced chemical changes in the rhizosphere (Dunbabin and Bowmer, 1992). Plants also provide habitat and energy sources to maintain and stimulate a diverse microbial population in the sediments (National Rivers Authority, 1992; Brodie, 1993; Skousen et al., 1994). These microbes drive the immobilization of contaminants in the sediments through both oxidative and reductive processes (Johnson, 1998).

In 1995, the Electric Power Research Institute (EPRI), in collaboration with Allegheny Power Services (APS), constructed a wetland treatment system to treat the leachate from a coal ash pile near the Springdale Electrical Power Station, Springdale, PA. The treatment system included an equalization basin, four vegetated wetland cells, two rock drains, an anaerobic (organic reduction) cell, and an algal cell (Electric Power Research Institute, 1998). The project was implemented to test the efficiency and economic feasibility of constructed wetlands for the treatment of coal combustion by-product leachate. The treatment system was designed particularly to achieve regulatory compliance for different trace metal contaminants (e.g., with the NPDES limits of 1.6 mg L-1 for Fe and 1.1 mg L-1 for Mn) before the leachate was discharged to a tributary of the Allegheny River. The present study focuses on the four vegetated wetland cells to determine (i) how effectively the wetland cells removed iron (Fe), manganese (Mn), cobalt (Co), and nickel (Ni) from coal combustion by-product leachate and (ii) the compartmentalization of these four elements in different sinks (e.g., sediments, plant tissues, etc.) within the vegetated wetland cells.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Study Site
The Springdale coal combustion by-product (mainly fly and bottom ashes) disposal site was established in the 1950s near the Allegheny Power Service in western Pennsylvania. As a valley-fill operation, the disposal area (approximately 20 ha) was subsequently taken off active status and revegetated in the 1970s. To treat metal-contaminated leachate discharged from the site underdrain, the Springdale surface-flow wetland system was constructed in 1995. It comprises an aerobic equalization basin and eight lined cells, each of approximately 9 x 36 m (Fig. 1). The wastewater flowed sequentially from the first treatment phase (equalization basin), through four fully vegetated wetland cells, two rock drains, an anaerobic (organic reduction) cell, and finally an algal uptake cell (Electric Power Research Institute, 1998). An early study commissioned by Electric Power Research Institute (1998) showed that the water influent to the equalization basin upstream of the wetland cells contained an average of 12.46 mg Fe L-1, 2.71 mg Mn L-1, and 0.063 mg Ni L-1 (Co was not measured). The oxygenated equalization basin was effective for Fe removal (about 76% of the total Fe in the leachate), but was less effective for Mn (14%) and Ni (24%) (Electric Power Research Institute, 1998).



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  		Fig. 1. The Allegheny Power Services (APS) treatment system for coal combustion by-product leachate.

 
The present study focuses on metal removal by the four vegetated wetland cells. These cells (Cells 1–4) are of approximately equal size, and have a total combined area of approximately 1300 m2. After construction, the cells were filled with a blended compost substrate, comprising one part spent-mushroom compost to two parts native soil material. The native soil was classified as Hazleton (loamy-skeletal, siliceous, subactive, mesic Typic Dystrudrept). The cells were filled with 45 to 60 cm of this substrate. The wetland cells were planted with commercially grown cattail rhizomes. Additional bulrush (Scirpus spp.) and sedges (Carex spp.) were added as seed. At maturity, cattail was by far the most dominant species, and constituted the majority of the biomass produced in the vegetated cells. A variety of invasive plant species, for example, arrowhead (Sagittaria latifolia Willd.), a native grass (Panicum dichotomiflorum Michx.), and smartweed (Polygonum pensylvanicum L.), and also algae, for example, muskgrass (Chara spp.) and Spirogyra spp. were found in the cells.

The wetland monitoring program by the University of California Berkeley began in May 1996 and continued to June 1998. Since the residence time of each vegetated wetland cell was very short (i.e., about 5 h) (Electric Power Research Institute, 1998), most of the other trace elements we studied, such as boron (B), aluminum (Al), copper (Cu), or zinc (Zn), were poorly removed by the vegetated wetland cells. In this paper, we present data for Fe, Mn, Co, and Ni, that is, those elements that were shown to be effectively removed from the wastewater by the vegetated wetland cells.

Water Sampling
Influent and effluent waters were sampled quarterly from May 1996 through May 1997, and then monthly from July 1997 to June 1998. The pH, temperature, electrical conductivity (EC), and dissolved oxygen (DO) content of the surface water of each cell were measured in situ at the wetland using a Corning Checkmate Modular System (Fisher Scientific, Pittsburgh, PA). Water samples for trace element analysis were collected from the inlet of Cell 1 and from the outlet of Cell 4 of the vegetated cells. In March, April, and June 1998, water samples were also collected from the outlet of Cells 1, 2, and 3. In each case, three replicate 250-mL grab samples were collected using the bottle submersion method (Byrnes, 1994), and acidified immediately with 1 mL of concentrated nitric acid. All samples were transported to the laboratory at 4°C for chemical analysis.

Samples of pore water in the 0- to 10-cm sediment layer were collected at the same time as the influent and effluent waters. Pore water was collected from the same quadrats as the vegetation sample, using 0.1-µm Rhizon soil moisture filters (Ben Meadows, Canton, GA) connected to 20-mL Vacutainer tubes (Becton Dickinson, Franklin Lakes, NJ), which were allowed to fill to capacity. This method provided a sample from the composite 0- to 10-cm sediment profile. The sample was transported to the laboratory at 4°C for chemical analysis.

Plant Sampling
Plant samples were collected on a monthly basis from May 1996 through June 1998. Plant species composition and plant species density were sampled in three randomly selected 1-m2 quadrats in each vegetated cell. Three replicate whole-plant samples (shoot and root) were collected from the quadrats, to give estimates of total plant biomass. Samples of algae were also collected whenever available. All plant samples were placed in polyethylene Ziploc bags and transported to the laboratory at 4°C. Plant samples were washed with tap water and rinsed with distilled water. Shoot and root tissues were separated, and the samples were dried at 70°C for 1 wk. The dried tissues were weighed and ground to pass a 40-mesh screen using a Wiley mill in preparation for chemical analysis.

Sediment Sampling
Sediments were sampled quarterly over the first year (May 1996 to May 1997), and an additional collection was made at the end of the study (November 1998). Three replicate sediment samples were collected from the top 15 cm of sediment in each cell using an AMS (American Falls, ID) sediment recovery probe with replaceable 5- x 25-cm butyrate liners. The sediments were transported to the laboratory in the capped liners at 4°C. Sediments were then removed, cut into three 5-cm profile sections, air-dried, and ground to a fine powder.

Chemical Analysis
All samples were analyzed for total trace elements. Water samples were digested with HNO3, H2O2, and HCl according to USEPA Method 3010A (USEPA, 1992). Trace element concentrations were determined by inductively coupled plasma atomic emission spectrometry (ICP–AES). Elemental analysis of plants was carried out by acid digestion of dry samples (Zarcinas et al., 1987), followed by measurement of total concentrations of all elements of interest in the acid digest using ICP–AES. Sediment samples were acid digested with HNO3, H2O2, and HCl using USEPA Method 3050B (USEPA, 1996) before chemical analysis by ICP–AES.

Statistical Analysis
Sample means and standard errors were determined from the replicate water, plant, and sediment samples. The data were analyzed statistically using one-way analysis of variance (ANOVA); significant differences between individual means were determined using the Tukey multiple comparisons test (Zar, 1984).


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Physical Characteristics of Surface Water
There were no significant differences (P > 0.05) between the four cells in the pH, EC, DO, and temperature of the surface water. The values are therefore averaged over the four cells (Fig. 2). Over the 26-mo study period, the pH of the surface water averaged 7.2, and ranged from 6.2 to 7.7. The average EC was 2.2 mS cm-1, ranging from 1.9 to 3.0 mS cm-1. There was some seasonal variation in the DO concentration in the surface water, with concentrations ranging from 5 to 14 mg L-1. The lowest values were in the summer (July through September), and the highest in the winter. The water temperature varied greatly from 0 to 30°C during a year. The DO and temperature were inversely proportional; high temperatures resulted in the lowest concentrations of DO (Fig. 2). The lowest DO values were most likely due to the lower solubility of oxygen in summer and to the greatest microbial activities when water temperature was high.



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  		Fig. 2. Temporal changes in the surface water pH, electrical conductivity (EC), dissolved oxygen (DO), and temperature. Values are averaged over the four vegetated cells for each month. Error bars indicate standard errors (SE). Dashed horizontal line denotes overall average.

 
Trace Element Concentrations in Inlet and Outlet Waters of the Vegetated Cells
The four vegetated wetland cells removed significant quantities of Fe, Mn, Co, and Ni from the wastewater; the best removal rates were seen for Fe and Mn. Greater reductions in the concentrations of the four elements were seen in the second year (i.e., after February 1997) compared with the first year. During the study period, the wastewater entering Cell 1 from the equalization basin contained as much as 4.69 mg Fe L-1, 3.3 mg Mn L-1, 0.022 mg Co L-1, and 0.055 mg Ni L-1 (Fig. 3). However, in the outlet water from Cell 4 the concentrations of Fe and Mn were low, ranging from 0.002 to 0.49 mg L-1 and 0.0005 to 0.44 mg L-1, respectively. Concentrations of Co and Ni in the outlet from Cell 4 were also low compared with their concentrations in the inlet water. The monthly reduction (%) in concentration shows that, in some months, almost 100% of these elements had been stripped from the wastewater by the vegetated wetland cells. When these percentages were averaged for the first year of the study, the concentrations of Fe, Mn, Co, and Ni were reduced by 90.8, 91, 38.5, and 47.3%, respectively. In the second year, the metal concentration was reduced by 94% for Fe, 98% for Mn, 96% for Co, and by 62.6% for Ni. Particularly, Cell 1 appeared to remove most of the elements from the wastewater by comparison with the removal by the other three cells (Fig. 4).



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  		Fig. 3. Temporal changes in the concentration of Fe, Mn, Co, and Ni in the inlet water to Cell 1 (hatched bars), and the outlet water from Cell 4 (filled bars). Values are means plus SE, n = 3.

 


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  		Fig. 4. Concentrations of Fe, Mn, Co, and Ni in the inlet water to Cell 1 and the outlet water from Cells 1, 2, 3, and 4 in 1998. Solid line denotes the percentage reduction in concentration. Values are means plus SE, n = 3.

 
Trace Element Distribution in Vegetated Wetland Cells
Accumulation in Sediments
The average concentrations of Fe and Mn accumulated in the top 15 cm of sediment in the four vegetated cells were 32300 and 1200 mg kg-1, respectively. In contrast, only 18 mg kg-1 Co and 41 mg kg-1 Ni were in the top 15 cm of sediments. Higher concentrations of all four metals were observed in the surface layers (0–5 cm) of sediment compared with the lower layers (5–10 and 10–15 cm) in Cell 1 (Table 1). Because the sediment was mixed soil that was deposited into each wetland cell when the wetland was constructed, the background metal distribution in sediments should be generally uniform prior to the beginning of this study. It is evident therefore that metals (Mn, Co, and Ni, less for Fe) from the inlet were being deposited in the surface layer of sediments. In Cells 2, 3 and 4, this stratification was not seen; all three layers had similar concentrations of each metal (Table 1).


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  		Table 1. Metal concentrations (mg kg-1) at different depths in the sediment profile. Data are means (±SE, n = 6) of samples collected from all four cells between May 1996 and November 1998. Differences between depths were determined by analysis of variance (ANOVA); letter codes indicate significant differences (Tukey multiple comparisons test, P < 0.05).

 
The concentrations of Mn, Co, and Ni in the top sediment were less in Cell 2 than in Cell 1, but changed little with increasing distance from Cell 2 to Cell 4. There was a significant increase in the metal concentration in the sediment of the four vegetated cells during the first year of operation (Fig. 5). The average concentrations of Fe in the sediment samples (0–15 cm) were typically more than threefold higher than the concentration of Fe in the substrate from which the wetland was constructed (e.g., 32500 mg Fe kg-1 in May 1996 compared with 9000 mg Fe kg-1 in November 1995). However, during the second study period (May 1996 through November 1998) there were no significant changes in the concentration of Fe in top 15 cm of sediment. The concentrations of Mn, Co, and Ni in the sediment (0–15 cm) increased with time from November 1995 to November 1997 (Fig. 5).



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  		Fig. 5. Concentrations of Fe, Mn, Co, and Ni in the top 15 cm of sediment in four vegetated wetland cells. The gray bars show the concentration of each element in the substrate before the water treatment began (data from Electric Power Research Institute, 1998). Values are monthly averages of all four cells plus SE, n = 4.

 
Sediment Pore Water
The average concentrations of the metals, except for Mn, in sediment pore water during the period of May 1996 to June 1998 were similar to their concentrations recorded in the inlet water (Fig. 3 and 6). The metal concentration of the pore water varied among the four vegetated cells. The concentrations of Mn, Co, and Ni (but not Fe) in pore water tended to decrease with increasing distance from Cell 1 through to Cell 4 (Fig. 6).



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  		Fig. 6. Concentrations of Fe, Mn, Co, and Ni in sediment pore water (0–10 cm depth) in each vegetated wetland cell (May 1996 to June 1998). Values are means plus SE, n = 16.

 
Accumulation in Cattail, Fallen Litter, and Chara
The average concentrations of Fe, Mn, Co, and Ni in cattail shoots and belowground tissues varied considerably for each element during the study period (May 1996–June 1998) (Table 2). The greatest metal concentration in the shoots of cattail was Mn, followed by Fe > Ni > Co. In the roots, however, the highest concentration was seen for Fe, followed by Mn > Ni > Co. The concentration of Co in the shoots and roots of cattail from Cell 1 were higher than those in the plants growing in the other three cells. The Fe, Mn, Co, and Ni contents of cattail fallen litter were much greater than for the living shoots of cattail; in the case of Mn, Co, and Ni they contained higher concentrations than even the belowground tissues (Tables 2 and 3). The metal concentrations in the fallen litter decreased from Cell 1 to Cell 4.


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  		Table 2. Metal concentrations (mg kg-1) in the aboveground and belowground tissues of cattail collected from the four cells. Values are means (±SE, n = 26) of samples collected between May 1996 and June 1998. Differences between cells determined by analysis of variance (ANOVA); letter codes indicate significant differences (Tukey multiple comparisons test, P < 0.05).

 

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  		Table 3. Metal concentrations (mg kg-1) in the fallen litter of cattail and in Chara, a macroalga. Fallen litter samples were collected from the four cells, during the study period of November 1996 to March 1997, and November 1997 to March 1998; Chara samples were collected from Cells 1 and 4 during the period of May 1996 to February 1998. Values shown are means and one standard error. Means with different letters indicate significant differences (Tukey multiple comparison test, P < 0.05).

 
The average concentrations of all four metals on or in the living tissues of Chara were very high, considerably higher than for both the above- and belowground tissues of cattail (Tables 2 and 3). Indeed, the concentrations of Co and Ni in Chara were about 238 and 173 times higher than that in the aboveground tissues of cattail, respectively. In addition, the concentrations of Fe, Mn, Co, and Ni in Chara growing in Cell 1 were dramatically higher than those of plants grown in Cell 4 (Table 3).

Loading and Retention of Trace Elements in Vegetated Cells
When the difference between the total metal loading at the inlet of Cell 1 and the unloading of metals at the outlet of Cell 4 was calculated, it showed that most of the metals had been retained within the cells (Table 4). On a per unit area basis, approximately 68 g of Fe, 149 g of Mn, 0.58 g of Co, and 1.5 g of Ni per m2 were retained by the vegetated cells during the period of July 1997 through June 1998; these amounts account for 94.6, 98.8, 96.1, and 62.1% of the annual total Fe, Mn, Co, and Ni mass input to the vegetated wetland cells, respectively. Most of the metals measured were retained in the sediments; only small amounts were contained in cattail shoots, fallen litter, and Chara (Table 4). Cattail shoots contained much higher amounts of Mn (4.18% of the total Mn removed by the four cells) than the other three elements.


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  		Table 4. Annual metal budget for the vegetated cells of the wetland (July 1997–June 1998). Estimated metal loading and unloading rates calculated for the wetland cells. The retention and distribution of the metals in the cells are estimated on a unit area basis. The accumulation of metals in the plants is also shown as a percentage of the metals removed by the vegetated cells.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Metal Removal Efficiency of the Vegetated Cells
The water flowing into Cell 1 of the vegetated part of the wetland contained Fe and Mn concentrations above the 1996 NPDES permissible limits. Previous studies showed that constructed wetlands are effective in removing Fe from metal-polluted waters, but are less successful in removing Mn (Stillings et al., 1988; Fennessy and Mitsch, 1989; Hedin, 1989; Skousen et al., 1994; Stark et al., 1994). Hedin (1989) noted that only 2 sites of 41 studied with inflow Mn concentrations greater than 2.0 mg L-1 achieved compliance with the standard. Our study demonstrates that the four vegetated wetland cells successfully removed a large proportion of both the Fe and Mn loads from the inlet water; the concentrations of Fe and Mn were decreased by an average of 91% in the first year (May 1996–May 1997), and by 94 and 98% in the second year (July 1997–June 1998), respectively. The vegetated cells also decreased the concentrations of Co and Ni but to a lesser extent (i.e., by an average of 38.5 and 47% in the first year, and 97.5 and 62.6% in the second year, respectively).

The solubility of Fe and Mn in water is governed mainly by their oxidation state, which in turn is affected by pH. Skousen et al. (1994) suggested that ferric hydroxides precipitate as pH increases above 3.5, while Mn hydroxides require a pH of at least 7 to precipitate. In our study, the surface water in the wetland cells had a circumneutral pH (average 7.2, range 6.2–7.7). This neutral pH may therefore have been a significant factor contributing to the effective removal of Mn by the wetland cells. In an investigation of 14 constructed wetlands, Brodie (1993) highlighted the apparent correlation between Mn removal and the alkalinity and/or acidity of the influent water; it was found that Mn loading into the 14 wetlands ranged from 0.17 to 2.00 g m-2 d-1, and that the Mn unloading ranged from 0.15 to 1.87 g m-2 d-1, corresponding to a 1 to 96% Mn removal. Of the systems with low removal rates, all were associated with low pH in the wastewater (2.9 to 3.9). Hedin and Nairn (1993) also indicated that the alkalinity of mine water is a major determinant in the kinetics of contaminant removal by wetlands. Whereas Fe removal rates increased with increasing pH, the removal of Mn occurred only under alkaline conditions. Therefore, the superiority of our vegetated wetland cells in removing Mn was almost certainly associated with a high water pH value (i.e., > pH 7.2).

Metal removal by the APS wetland was sustained throughout the whole year. This is in contrast to some other studies that found distinct seasonal variation in removal rates. Faulkner and Skousen (1994) showed that climate, season, and nutrient availability dramatically affect the amount of sulfide generation and subsequent metal reduction. Henrot and Wieder (1990) noted that Fe removal efficiency by peat increases linearly with temperature in the 4 to 25°C range; they suggested that the performance of constructed wetlands in treating trace element–contaminated water varies seasonally, with a higher efficiency in the summer. The results of our study, however, showed that the vegetated cells decreased the concentrations of metals by similar amounts throughout the entire year (July 1997 through June 1998) (Fig. 3), despite low temperatures in winter. This might be attributable to the relatively constant pH value with season and to the higher dissolved oxygen level in winter, both of which favor oxidation and the formation of insoluble hydroxides of metal elements. The fact that the removal rates at the APS wetland were not linked to seasonal changes is very beneficial because it ensures that discharges are in compliance with regulatory limits throughout the year.

Distribution of Metals within Sediments and Pore Water
The concentrations of Fe, Mn, and Ni in the sediments of the APS wetland (Table 1) were comparable with those of other cattail-dominated treatment wetlands, for example, 180 to 84700 mg Fe kg-1, 31 to 2620 mg Mn kg-1, and 1 to 24300 mg Ni kg-1 (Babcock et al., 1983; Taylor and Crowder, 1983; Sencindiver and Bhumbla, 1988; Meiorin, 1989; Fernandes and Henriques, 1990). However, the concentrations of Mn, Co, and Ni in the top layer (0–5 cm) of the sediments in Cell 1 were higher than their concentrations in the lower layers (5–10 and 10–15 cm) of the same cell (Table 1). The surface layers of sediment are generally aerobic, facilitating metal removal via the formation of hydrated metal oxides, and have high microbial activity, important for the chemical transformation of metals in sediment pore water (Hedberg and Wahlberg, 1998).

There was a decrease in the concentration of metals (except Fe) in sediment with increasing distance from the inlet. The concentrations of the metals were greatest in Cell 1, and least in Cell 4. Similar findings have been reported for Fe, Mn, and other trace elements such as As and Zn by several investigators (e.g., Beining and Otte, 1996; Keller et al., 1998; Mitsch and Wise, 1998). Except for Fe, the concentrations of Mn, Co, and Ni in the sediment pore water also tended to decrease from Cell 1 to Cell 4 (Fig. 6). Beining and Otte (1996) found a similar reduction in the metal (Zn) load of the pore water with increasing distance from the inlet.

Accumulation of Metals by Plants and Fallen Litter: Role of Vegetation
To effectively use constructed wetlands for metal removal, it is important that the wetland vegetation can tolerate the levels of potentially toxic metals present in the wastewater. In our study, the growth of cattail was not detrimentally affected by the metals present, despite the high concentrations of metals in the water and sediment. The maximum aboveground biomass was 2800 g dry weight m-2, comparable with 2900 g m-2 for healthy cattail growing in a noncontaminated Texas pond (Hill, 1987).

Although plants play a critical role in the removal of metals by constructed wetlands through physicochemical processes and by supporting microbial activity (Kadlec and Knight, 1996), the uptake of metals into living shoots represented only a minor component of the metal removed by the APS wetland. In the present study, for example, only 0.91, 4.18, 0.19, and 0.38% of Fe, Mn, Co, and Ni were accumulated annually in the aboveground tissues of cattail (the dominant species of the APS wetland) per m2, respectively (Table 4).

Considerably higher metal concentrations were measured in the fallen litter of cattail than were present in the living cattail shoots (i.e., 3, 13, 85, and 28 times higher for Fe, Mn, Co, and Ni, respectively). High concentrations of metals were also accumulated by the living tissues of Chara, a macroalga growing primarily in Cells 1 and 4. Other researchers have noted that submerged plant tissues, including Chara, may constitute a considerable sink for metal removal in wetlands (Reimer and Toth, 1968; Kepler, 1988; Sparling and Lowe, 1998). It is possible that the high concentrations of metals associated with both fallen litter and with Chara were due partly to the fact that dead plant material and living Chara tissues provide adsorption sites and sites of precipitation for metal accumulation (Skousen et al., 1994; Horne, 2000). The concentrations of all four metals in cattail fallen litter and Chara collected from Cell 1 were higher than those collected from Cell 4, respectively, probably as a direct consequence of the higher concentration of metals in the surface water of Cell 1 compared with Cell 4. The accumulation of Fe, Co, and Ni in fallen litter and submerged Chara tissues together accounted for a significantly greater proportion of the metals removed by the wetland (i.e., 2.81, 2.75, and 1.05% for Fe, Co, and Ni, respectively) than the proportion removed by living cattail shoots. Interestingly, living cattail shoots accumulated greater amounts of Mn than fallen litter and Chara, that is, 4.2% compared with 1.8% of Mn removed (Table 4).

Cattail plants play an important role in metal retention by virtue of the immobilization of metals in the oxygenated rhizosphere (Otte et al., 1995; Cacador et al., 1996). Doyle and Otte (1997) reported that oxidation processes and Fe concentrations were consistently higher in the rhizosphere than in the bulk soil for all the plant species they studied. Assuming that the biomass of cattail roots and rhizomes are equal to those of the shoots (Sencindiver and Bhumbla, 1988; Vymazal, 1995), we estimate that the belowground biomass of cattail in the APS wetland could have taken up as much as 20.2 g Fe m-2. This represents 29.7% of the total amount of Fe retained by the vegetated cells compared with 0.91% for the shoots in the same area. Part of this Fe was almost certainly associated with iron plaque, which may be up to 15 to 17 µm thick on cattail root surfaces (Taylor et al., 1984). Our research has shown that there may be approximately ninefold more Fe on the surfaces of cattail roots than is accumulated within the root tissues (Ye, unpublished data, 1999). Manganese may also be accumulated to high concentrations in the iron plaque (Ye et al., 1997).

Our conclusions are in clear agreement with previous studies (e.g., Sencindiver and Bhumbla, 1988; Dunbabin and Bowmer, 1992; Mitsch and Wise, 1998) that indicate that the substratum (sediment) is the primary sink for metals in treatment wetlands. Metal accumulation in the APS wetland, Pennsylvania, tended to be greater in the surface layers of sediments as well as in the rhizomes of cattail. The accumulation of metals in living shoot tissues of cattail (the dominant plant species in the wetland), and the accumulation in cattail fallen litter and submerged living Chara tissues, were relatively minor sinks in comparison with the sediments. We conclude that the vegetated constructed wetland at Springdale is a highly effective, low-maintenance treatment system for cleaning Fe and Mn in coal combustion by-product leachate to comply with the NPDES permits.


   ACKNOWLEDGMENTS
 
Funding for this study provided by the Electric Power Research Institute (EPRI) (4163-01), Palo Alto, CA. The authors thank Dr. M. de Souza for reviewing this manuscript.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Z.H. Ye, present address: Institute for Resource and Environmental Management, Hong Kong Baptist University, Hong Kong.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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